CVE-2026-68147: fscrypt: Avoid dynamic allocation in fscrypt_get_devices()

In the Linux kernel, the following vulnerability has been resolved:

fscrypt: Avoid dynamic allocation in fscryptgetdevices()

When a blkcryptokey starts being used or is evicted, fs/crypto/ calls fscryptgetdevices() to get the filesystem's list of block devices, then iterates over them and calls blkcryptoconfigsupported(), blkcryptostartusingkey(), or blkcryptoevictkey() on each one.

Currently, the block device pointers are placed in a dynamically allocated array. This dynamic allocation is problematic because:

- It can fail, especially at the fscryptdestroyinlinecryptkey() call site when it's invoked for inode eviction under direct reclaim.

- fscryptdestroyinlinecryptkey() doesn't handle the failure. It just zeroizes and frees the blkcryptokey without calling blkcryptoevictkey(). That causes a use-after-free.

For now, let's fix this in the straightforward and easily-backportable way by switching to an on-stack array. Currently the fscrypt multi-device functionality is used only by f2fs, which has a hardcoded limit of 8 block devices. An on-stack array works fine for that.

(Of course, this solution won't scale up to large number of block devices. For that we'd need a different solution, like moving the block device iteration into the filesystem. Or in the case of btrfs, which will only support blk-crypto-fallback, we should make it just call blk-crypto-fallback directly, so the block devices won't be needed.)